(a) Field of the Invention
The present invention relates to an objective lens for microscopes.
(b) Description of the Prior Art
When numerical aperture(NA) exceeds 0.3 or 0.4 in an objective lens for microscopes, it comes to pass that variation of aberration attributed to a plane parallel glass(i.e., a cover glass) provided between a lens and an object cannot be neglected. Like the case where cellular tissue is observed through the bottom of a cultivating vessel in the field of tissue culture in particular, if the thickness of the plane-parallel plate is approximately 1 mm and varies greatly throughout the whole, difficulties arise that, with common lenses, the variation of aberration is considerable and an image is disturbed. Thus, a method has been known and is often used at present that proper lens unit is selected to be moved along an optical axis by a cam interlocking with a correcting ring, thereby canceling the variation of aberration.
Further, in a phase-contrast microscope or a modulation-contrast microscope, a stop having a particular configuration is arranged in an illuminating system and an optical modulator such as a phase ring(for the phase-contrast microscope) having size corresponding to the aperture of the stop or such as a density pattern(for the modulation-contrast microscope) is provided at an pupil position of an objective lens to modulate part of light transmitting an object so that an object invisible through an ordinary microscope can be observed. This, however, has encountered a problem that, since the optical modulator is fixed at the rear of spaces between the lens units used for correction in a conventional objective lens, the optical modulator is out of the pupil position of the objective lens when the lens units are partially moved by the correcting ring to change the spaces between the lens units.
Although the system of such a type that the optical modulator is out of the pupil position like the conventional obJective lens therefore makes allowance for the size of the optical modulator so as to be capable of performing completely the modulation even in a state of deviation from the pupil position, this has brought about another problem that, when placed at the pupil position, the optical modulator is so enormous in size that light unnecessary for modulation is also modulated and the contrast of the image deteriorates.